1. Field of the Invention
This invention relates generally to the fabrication of a MR sensor. In particular it relates to an MR sensor in which a hard bias layer can be reduced in thickness and enhanced in effect by means of a flux guide structure.
2. Description of the Related Art
With the ever increasing areal density with which data is stored on magnetic media such as disks in hard disk drives (HDD), the magneto-resistive (MR) sensor that is used as the read-back element in the HDD is required to have a correspondingly improved spatial resolution while achieving and maintaining a reasonable signal-to-noise ratio (SNR). Referring to schematic FIGS. 1a, 1b and 1c, there are shown three views of a generic, prior-art current-perpendicular-to-plane (CPP) MR read head. FIG. 1a illustrates the read head in a vertical cross-sectional plane parallel to its air bearing surface (ABS) plane. FIG. 1b illustrates the read head from an overhead view of a horizontal cross-sectional plane through its magnetically free layer (discussed below). FIG. 1c is a portion of the illustration of FIG. 1a, isolating the sensor stack portion of the head.
Referring to FIG. 1a, there is shown the CPP MR head, which could be a CPP-GMR head (current perpendicular-to-plane giant-magneto-resistive head), in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the giant-magneto-resistive effect. Alternatively, the head could be a CPP-TMR (current perpendicular-to-plane tunneling magneto-resistive) head, in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the tunneling-magneto-resistive effect. Either of these particular types of head, which are state-of-the-art read-back heads, will be well represented by the discussion that follows.
FIG. 1a shows the following physical elements of the generic prior art head. Looking vertically downward, there is first shown an upper (or top) shield (1) that protects the sensor stack (6) from extraneous magnetic fields. At the bottom of the head, there is shown a corresponding lower (or bottom) shield (2) that performs a similar task at the bottom edge of the sensor. Thus the sensor is protected by a pair of shields at some desired separation (3).
Hard bias (HB) magnets (4) (magnets formed of hard magnetic material) are laterally disposed to either side of the sensor stack (6). These magnets, which stabilize the magnetization of the free layer (8) are positioned between the shields (1), (2) and their magnetizations are shown as arrow (5). These hard magnetic layers are formed on underlayers (20) that promote the requisite crystalline anisotropy. The sensor stack itself (6) is typically formed as a patterned lamination of five horizontal layers, formed beneath an upper capping layer (18). An arrow (7) shows the direction of magnetization of the magnetically free layer of the sensor stack, as seen in FIG. 1c. 
FIG. 1b is a horizontal cross-sectional slice through the two HB layers (4) and the magnetically free layer (8) of the sensor stack, as will be discussed below.
Referring to FIG. 1c, there is shown a schematic, illustration of the isolated sensor stack (6) of FIG. 1a showing the following five horizontal layers: the magnetically free layer (8), showing it magnetization vector as an arrow (7); a layer (9) that is a dielectric layer that serves as a tunneling barrier layer for the TMR sensor, or is a conducting layer (9) for the GMR type sensor, a reference layer (10), a coupling layer (eg. a layer of Ru) (11), a pinned layer (12) whose magnetization is held spatially fixed by a thick layer (19) of antiferromagnetic material that also pins layer (10). The hard biasing layers (4), with longitudinal magnetization (5), provides a biasing magnetic field in the sensor stack (6) to orient the magnetization (7) of the free layer (8) in a longitudinal direction. A capping layer (18) is positioned between the free layer (8) and the upper shield (1). In forming the sensor, the stack and the hard bias layers are defined by a single etching process that insures they are at the same height.
In the most modern disk drives, the height at which the head flies above the rotating disk is already less than 5 nm, so the freedom of further flying height reduction to increase spatial resolution is reaching its limit. Thus, the common practice to increase the resolution is by reducing the reader-shield-spacing (RSS) (3), so the magnetic spatial resolution increases correspondingly.
To reduce RSS, the thickness of the hard bias (HB) layers (4) will need to decrease as well. However, reducing the thickness of these magnetic HB layers will also reduce the amount of magnetic “charge” at the edges of the HB layers immediately adjacent to the sensor stack and facing the edges of the free layer. The fewer the magnetic charges, the less is the pinning field of the HB layers and the less effective they are at orienting the free layer magnetization. Meanwhile, with a much smaller spacing between the HB layers and the upper read shield (1), magnetostatic coupling between the HB layer and the upper shield is increased, which can rotate the magnetization of the HB layers away from their desired orientation at the free layer edges. Thus the effective field of the HB layers at the free layer is degraded by these two effects, less magnetic charge and magnetostatic coupling to the upper shield.
Studies (see: Y. Zhou, “Thermally Excited Low Frequency Magnetic Noise in CPP Structure MR Heads,” IEEE Trans. Magn., vol 43, pp 2187, 2007) show that a weakened HB field increases the noise produced by the sensor and will ultimately affect the reading process in high density magnetic recording. To increase the HB field for thinner HB layers, a higher HB magnetic moment and/or closer HB to free layer spacing is required. To reduce the effects of magnetostatic coupling between the HB layers and the reader shield, common practice is to increase the coercivity of the HB material (D. J. Larson et al., U.S. Pat. No. 7,061,731 B2; P. V. Chau et al., US Publ. Pat. Appl. 2005/0066514; H. S. Gill, US Publ. Pat. Appl. 2006/0114622 A1; M. M. Pinarbasi, US Publ. Pat. Appl. 2006/0087772 A1) so that the HB magnetization does not easily rotate because of the magnetostatic coupling to the upper shield. However, increasing the magnetic moment and increasing the coercivity of the HB materials are contradictory procedures according to the physics of magnetic materials. Increasing one property decreases the other. Thus, a high moment, high coercivity HB material is difficult to achieve so that it produces enough HB field on the free layer and is stable enough at narrow RSS.
In addition to the higher spatial resolution of the MR sensor in the down-track direction that requires a narrower RSS, higher area density of recorded data requires a higher track density so that the data tracks can be recorded more closely together. The track width will therefore be reduced, which also requires the reader to be narrower. For the conventional HB layer, as the width of the sensor stack diminishes, the distance between the HB and the stack edges does not diminish proportionally, because of the minimum interlayer distance that is required between the stack edges and the HB layer edges. This interlayer includes both a nonconductive layer that electrically isolates the HB layer from the sensor stack and an underlayer between the HB layer and the nonconductive layer to promote the crystalline growth of the HB layer in order to maintain a high coercivity. According to the prior study of Y. Zhou, cited above, this limitation on the minimum HB-to-sensor stack distance leads to a lower HB field gradient from the free layer edge to the free layer center. A lower HB gradient makes the MR sensor either have a higher noise production at the same sensitivity or a lower sensitivity in order to maintain the same noise level. Either of these alternatives leads to a lower signal-to-noise (SNR) ratio as sensor width is reduced.
The most ideal structure for an HB layer at narrow RSS and a narrow sensor width, is a thin and high-moment HB layer positioned as closely as possible to the free layer edge. This will only produce enough bias field to quench the self-demagnetization field of the free layer edge and, thereby, reduce random fluctuations in the free layer magnetization while producing much less field in the center of the sensor to avoid reducing sensor sensitivity.
To achieve a high moment in the HB layer, while still maintaining its magnetic stability, M. Arasawa et al., US Publ. Pat. Appl. 2006/0158793 A1 have suggested a dual-HB layer design as illustrated in FIG. 2a. A first pair of HB layers (13) are positioned at the sides of the sensor stack (6). A second pair of HB layers (4) are positioned laterally outside of layers (13). The inner HB layers (13) have a higher magnetic moment than the outer HB layers (4) and produce a high HB field in the free layer (8) of the stack (6). However, as the higher moment material of HB layer (13) usually also has a lower coercivity, the outer HB layers (4) are formed of material with lower moment but higher coercivity and are used to stabilized the inner layers (13) by applying their magnetic fields to the inner layers (13). This prior art, however, has the following limitations.                (1) The prior art proposes that the inner HB layer (13) be formed of a hard magnetic material similar to that of HB layer (4), but with a higher moment, which makes the underlayer between HB (13) and the sensor stack (6) still indispensable, so the distance between (13) and (6) cannot be further reduced.        (2) The prior art specifies that the product Mst (magnetic moment, Ms, times thickness, t) of the outer HB layer (4) needs to be greater than the same product of HB (13), so that HB (13) can be fully magnetized by HB (4). Although such a relationship can help maintain a full saturation of HB (13) magnetization at the edge facing FIB (4), it may not be able to prevent the magnetostatic coupling between of HB (13) to the upper shield (1) at the edge where HB (13) faces the sensor stack (6), because the HB (4) field produced in HB (13) decreases in inverse proportion to the distance from the HB (4) edge facing HB (13).        (3) The prior art also notes that HB (13) can be a soft magnetic material. However, it lacks details as to how one makes a soft HB (13), with coercivity approximately 0, to work in the structure shown in FIG. 2a. Since the prior art does not specify any additional features, except that HB (4) has a higher Mst, to make a soft HB (13) work, it is only logical to deduce that the prior art assumes a commonly adopted approach in forming the HB (13) on the sides of the sensor stack (6) and defining the back edges of the free layer and the HB layer in a single step etching process, leading to the top view shown schematically in FIG. 2b. A soft HB layer (13) as in FIGS. 2a and 2b would be difficult to stabilize by the field of HB (4) because the soft HB (13) can have a much higher moment and a lower coercivity. Coupling to the upper shield and perturbation by external fields will be much stronger. In addition, with the same uniform stack height of the soft HB (13) and the free layer, coupling between the free layer and the HB (13) also exists during read-back and can lead to large amounts of side reading. FIG. 3 displays simulated cross-track read-back profiles of a generic HB MR sensor and a FIG. 2a type structure MR sensor with HB (13) being formed of soft material. The coupling between the free layer and the HB layer (13) leads to significant side reading, where HB (13) is rotated by the field of the medium from side tracks and the magnetization of the free layer rotates as a result of coupling with HB (13).        
The present invention will address the problems alluded to above by achieving an optimum hard bias field on the free layer of a MR sensor stack at a narrow reader shield spacing (RSS).